Prolonged depression of knee extensor torque complexity following eccentric exercise

Abstract

Neuromuscular fatigue reduces the complexity of muscle torque output. Exercise-induced muscle damage reduces muscle torque output for considerably longer than high-intensity fatiguing contractions. We therefore hypothesized that muscle damaging eccentric exercise would lead to a persistent decrease in torque complexity, whereas fatiguing exercise would not. Ten healthy participants performed five isometric contractions (6 s contraction, 4 s rest) at 50% maximal voluntary contraction (MVC) before, immediately after, 10, 30 and 60 minutes, and 24 hours after eccentric (muscle damaging) and isometric (fatiguing) exercise. Further measures were taken 48 hours and one week after eccentric exercise. Torque and surface EMG signals were sampled continuously. Complexity and fractal scaling were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA). Global, central and peripheral perturbations were quantified using MVCs with femoral nerve stimulation. Complexity decreased following both eccentric (ApEn, mean (SD), from 0.39 (0.10) to 0.20 (0.12), P < 0.001) and isometric exercise (from 0.41 (0.13) to 0.09 (0.04); P < 0.001). After eccentric exercise ApEn and DFA α required 24 hours to recover to baseline levels, compared to only 10 minutes following isometric exercise. MVC torque remained reduced (from 233.6 (74.2) N.m to 187.5 (64.7) N.m) and submaximal EMG amplitude increased (from 51.2 (6.9)% to 68.4 (11.3)%) 48 hours after eccentric exercise, with such changes only evident up to 60 minutes following isometric exercise (MVC torque, from 246.1 (77.2) to 217.9 (71.8) N.m; submaximal EMG from 52.9 (6.4)% to 66.2 (9.0)%). The prolonged depression in maximal muscle torque output is therefore accompanied by a reduction in torque complexity, suggesting that eccentric exercise diminishes motor control as well as muscle force-generating capacity.

Complexity of Knee Neuromuscular Control in Uninjured Physically Active Adults: Right/Left and Dominant/Nondominant Asymmetry Analyses

“…26,27 Indeed, low complexity has been speculated to reflect a narrowing of system responsiveness and lower adaptability, which could increase the risk of failing a motor task. 28 Complex fluctuations result from multiple physiological components operating together to generate a whole-system output. 9,12 In neuromuscular control, motor pathways combine multiple neuroanatomical and physiological components including the descending tracts formed by upper motor-neurons originating in the cerebrum and brainstem and lower motor-neurons originating in the spinal cord.…”

“…6 If adaptability in knee extensor neuromuscular control differs between sides, the difference could originate from structural and/or physiological side-to-side differences in any component of a motor pathway on either side of the body. For example, it has been argued that changes in complexity are a consequence of variations in common synaptic input to the motor unit pool, 25,28 which are themselves the main determinant of the magnitude of force fluctuations. 29 Increased common synaptic input, and motor unit synchronization (a necessary consequence of common synaptic input 29 ), have been speculated to be associated with lower complexity.…”

“…29 Increased common synaptic input, and motor unit synchronization (a necessary consequence of common synaptic input 29 ), have been speculated to be associated with lower complexity. 28 Indeed, differences in motor unit synchronization have been observed between the muscles of the dominant/nondominant arms. 30 The present study had several potential limitations.…”

Individualized Whole-Body Vibration: Neuromuscular, Biochemical, Muscle Damage and Inflammatory Acute Responses

“…57 Consequently, the decrease in contribution to force production from some damaged motor units is compensated for by means of increased recruitment and rate coding, as evidenced by the EMG amplitude increase, to achieve the required level of torque. 58 The decrease in maximal force-generating capacity after exercise-induced muscle damage persists for several days, while after either isometric or concentric exercises, it recovers almost completely within 10 to 60 minute. 58 Since WBV involves a stretch-shortening cycle (SSC), 1,54,55 it is not surprising that the eccentric component of the SSC muscle action causes muscle damage.…”

“…58 The decrease in maximal force-generating capacity after exercise-induced muscle damage persists for several days, while after either isometric or concentric exercises, it recovers almost completely within 10 to 60 minute. 58 Since WBV involves a stretch-shortening cycle (SSC), 1,54,55 it is not surprising that the eccentric component of the SSC muscle action causes muscle damage.…”